1. Field of the Invention
The present invention relates to a light (or optical) transmitter, and in particular to a light transmitter comprising an automatic bias control circuit which automatically controls biases for external modulators which respectively modulate light inputs of various wavelengths such as a wavelength division multiplex (WDM) system.
For light modulations in a light transmitter, there are a direct modulation, an internal modulation, and an external modulation, which are currently used in relation to the kind of a laser as a light source and a communication rate etc.
In the direct modulation, the higher the transmission rate becomes the more the light signal is influenced with chirping, and therefore a long distance transmission becomes difficult due to wavelength distributions within an optical fiber.
Then, external modulators such as a Mach-Zehnder type external modulator made of LiNO.sub.3 where the chirping is not fundamentally caused or an electric field absorbing modulator have been got much attention.
For the stabilization of a light communication system with such an external modulator for a long term, it is required to stabilize optical signals against thermal variation and aging.
Moreover, as the capacity of communications are large-scaled in recent years, a technology in which the capacity is enhanced which can be transmitted via a single optical fiber by differentiating the wavelengths of modulated optical signals and multiplexing them as in the WDM system for optical communications at a high speed is applied to the light transmitter.
Therefore, it is now demanded that the technology which stabilizes the transmission of an optical signal can be applied to the WDM technology without problems.
2. Description of the Related Art
FIG. 19 shows the prior art light transmitter in the Japanese Patent Laid-open Publication No. 3-251815 or its U.S. counterpart U.S. Pat. No. 5,170,274 entitled "Optical Transmission". The disclosure of this U.S. Pat. No. 5,170,274 is incorporated herein by reference. A light source 1 provides its light output signal for an external modulator 2. The external modulator 2 has a modulation input terminal 2a and a bias input terminal 2b. To the modulation input terminal 2a is supplied a low frequency superimposed signal (driving signal) in which an input signal (logic signal) is superimposed at a driver 3 with a reference low frequency signal from an oscillator 4.
To the bias input terminal 2b is supplied a bias voltage provided by an automatic bias control circuit 11 (ABC loop) composed of an output terminal of the external modulator 2, a branch device 5, a photoelectric transducer 6, a filter (BPF) 7, an amplifier 8, a synchronous detector 9, and a bias controller 10.
In this automatic bias control circuit 11, the branch device 5 takes a part of the light output signal out of the external modulator 2 as a monitor light 1 and transduces it into a monitor signal 2 which is an electric signal and supplied to the filter 7. The filter 7 takes only a low frequency component, out of the monitor signal 2, which corresponds to the reference low frequency signal from the oscillator 4 and is sent to the amplifier 8 which generates an amp. output signal 5 amplified with a constant amp. gain.
The synchronous detector 9 compares the reference low frequency signal from the oscillator 4 with the phase of the amp. output signal 5 and sends a signal corresponding to the phase difference to the bias controller 10. The bias controller 10 provides an operating drift correction signal 6 for the bias input terminal 2b of the external modulator 2.
That is, an operating point drift is caused in the input/output characteristic of the external modulator 2 due to temperature changes and/or aging, etc, resulting in the quenching ratio of the light output signal being deteriorated The automatic bias control circuit 11 detects the frequency component of the low frequency signal output superimposed with the light output signal of the external modulator 2, compares it in phase with the reference low frequency signals from oscillator 4, and detects the drifting direction of the operating point. The operating point of the external modulator 2 is controlled according to the drifting direction, thereby compensating the operating point drift (automatic bias control).
The above-mentioned prior art light transmitter has a fixed combination of a light source and an external modulator. Once a set point of the luminescence level of the light source is adjusted so that the automatic bias control circuit may be operable, there is no need to re-adjust the set point.
That is, the prior art automatic bias control circuit is designed according to a light level at which the laser emits light.
Also, the automatic bias control circuit employs a photo device and an amplifier. Unless these circuit elements are operated linearly, the operating point of the external modulator is not correctly set. The reason is that a nonlinear phase variation and in turn a corresponding set point error are caused when the circuit operates nonlinearly because the automatic bias control circuit controls the phase relation of a modulating signal of e.g. 1 kHz detected with the synchronous detector.
Therefore, in the prior art light transmitter, the gains of the photo device and the amplifier in the automatic bias control circuit are constant so that the allowance of the automatic bias control circuit with respect to the optical level variation of the light source is narrow.
On the other hand, the following problems have now been found upon practical uses of such a circuit as the light transmitter in the WDM system where a lot of light sources with different wavelengths are combined with the external modulators.
FIG. 20 shows an example of the prior art light transmitter applied to the WDM system In this figure, reference numerals 1-1.about.3-3 denote laser light sources (LDs), 2-1.about.3-3 external modulators, 3-1.about.3-2 driving circuits (drivers), and 11-1.about.11-3 automatic bias control circuits (ABC loops), respectively. The LDs 1-1.about.1-3 emit light with different wavelengths .lambda.1.about..lambda.3.
Thus, in case that the luminescent wavelengths of the LDs 1-1.about.1-3 are differentiated, it is necessary to change bias currents applied to the LDs1-1.about.1-3. Therefore, the optical output levels of each of the LDs1-1.about.1-3 are to be different.
In this case, the amplitudes of monitor signals obtained respectively from the outputs of the external modulators 2-1.about.2-3 are different from each other according to the difference of those optical levels.
As a result, low frequency (e.g. 1 KHz) signal components in the monitor signals are influenced by level variations of the light sources because the low frequency signal components are modulated with a modulation factor, e.g. 5% with respect to that of the input signal.
That is, while the modulating signal (driving signal) from the driver is converted into an optical modulating signal by the external modulator, the amplitude of the optical modulating signal changes naturally according to the optical level of the light source. For example, when a light with the level of -3 dBm is inputted from the light source, H level of the optical modulating signal is 0 dbi, the average level -3 dBm, and L level 0 dBm (non-luminescence), respectively, in an ideal case where the loss of the external modulator is zero and the quenching ratio is infinity.
Therefore, when the low frequency signal for a gain control is superimposed with the input signal to produce a modulating signal (driving signal), the modulation factor, i.e. the amplitude of the low frequency signal component in the monitor signal will also change according to the optical level of the light source.
This will be described with reference to FIG. 21 illustrating the prior art operation.
The monitor signal 2 is provided as an output from the photoelectric transducer 6 in proportion to the level of the monitor light 1 from the branch device 5, as seen from FIG. 21A As the amplitude of the low frequency signal component of the monitor signal 2 varies, that of the output signal 5 of the amplifier 8 also varies, as seen from FIG. 21B.
Further, as the amplitude of the amp. output signal 5 varies, the operating point drift signal (compensation signal) 6 from the bias controller 10 also varies, as seen from FIG. 21C.
Thus, the detection of the monitor signal becomes difficult when the amplitude of the monitor signal varies smaller, where a normal automatic bias control is made impossible.
Therefore, when the prior art automatic bias control circuit is applied to the light WDM system, it is necessary to design the automatic bias control circuit individually corresponding to the level difference of the light sources, so that even if the amplitudes taken out of the low frequency superimposed signal are different from each other, the ABC circuit corresponding to the individual levels is realized.
Even if the ABC circuits are designed individually corresponding to the light sources, the output levels of the&lasers changes due to deteriorations etc. of the laser used for respective light sources. As a result, if the amplitude of the monitor signal becomes small, the level variation will be difficult to be dealt with.